As known in the art, gas-fired burners are used for non-propulsion applications including residential, business, logistics, and camp purposes. As used herein, the term “logistics” refers to military and/or battlefield operations; while the term “camp” or “camping” refers to civilian operations at locations lacking a power grid, for example, recreational, marine, rescue, refugee, and emergency operations where a power grid is temporarily out of service or where no power grid exists. As used herein, the term “gas-fired burner” refers to a heat-producing burner that generates heat through flame combustion of a fuel existing in a gaseous state of matter at standard atmospheric temperature and pressure. Such gaseous fuels include methane, natural gas, ethane, propane, and butane. The gas-fired burner is further characterized as “static” in that it does not involve materially significant moving parts or reciprocating motion, in contrast to a burner employed for propulsion purposes, such as those found in internal combustion engines and gas turbines. Static gas-fired burners are employed in incinerators as well as in commercial appliances, such as stoves, ovens, ranges, grills, griddles, stock pot burners, clothes dryers, hot water heaters, boilers, and the like.
Static gas-fired burners of the type found in commercial appliances combust gaseous fuels, such as natural gas, methane, ethane, propane, or butane. A supply of the gaseous fuel is required to be available at the location of the appliance. Transportation of the gaseous fuel to the required location is burdensome and costly, particularly under logistics and camp operations. Moreover, under certain circumstances transportation of gaseous fuels is restricted. In contrast, liquid distillate fuels, such as diesel and JP-8, are readily available at essentially all locations, including remote logistics and camp operations, as preferred fuels for propulsion purposes, namely, for transportation vehicles. Moreover, liquid distillate fuels have an advantage of a higher energy density per unit volume and further advantages in being less volatile and safer to handle, as compared to gaseous fuels. Consequently, it would be desirable to operate static gas-fired burners and commercial appliances utilizing static gas-fired burners on a readily available liquid fuel, such as diesel or JP-8, so as to avoid transporting a gaseous fuel to the location of the burner or appliance.
One problem with the above concept involves the fact that gas-fired burners are designed for a specific gaseous fuel at a designated supply pressure to achieve a select energy output. Variations in any of fuel composition, or fuel supply pressure, or a ratio of fuel to air supplied to the burner can produce variations in energy output. In turn, variations in energy output, for example those greater than about +/−10 percent, can produce undesirable effects, for instance, thermal inefficiency and flame instability, the latter evidenced by a flickering yellow flame. In addition, ignition of the fuel may be hampered. For this reason gas-fired burners used in commercial appliances are designed for use with a particular Wobbe Index gaseous fuel and cannot be operated on gaseous fuels having a significantly different Wobbe Index.
The Wobbe Index (Iw) or Wobbe number is an indicator of the interchangeability of fuel gases and is calculated as shown in the equation below:Iw=Vc/(Gs)1/2 where Vc is the heating value or calorific value of the gaseous fuel and Gs is the specific gravity of the gaseous fuel. Industry typically calculates a Higher Wobbe Index using a higher heating or higher calorific value of the fuel, wherein the higher heating or higher calorific value is defined as the gross heat output on fully combusting the fuel to carbon dioxide and water. A Lower Wobbe Index is calculated using the lower heating or lower calorific value of the fuel, wherein the lower heating or lower calorific value is defined as the gross heat output minus the heat of vaporization of water. Unless otherwise specified, Wobbe Index values provided herein refer to the Higher Wobbe Index. Moreover, any reference hereinafter to “low” or “high” Wobbe Index refers to relative numerical values of the Higher Wobbe Index. The specific gravity of the gaseous fuel is defined as a ratio of the density of the gaseous fuel compared to the density of a reference substance, specifically, the density of air, taken as 1.2 g/cm3 as measured at 20° C. and 101 kPa. The Wobbe Index is commonly expressed in British thermal units per normal cubic foot (BTU/scf) or megajoules per normal cubic meter (MJ/Nm3); and in this sense can be considered a measure of energy density. Typically, the Wobbe Index is not applicable to liquid fuels.
The Wobbe Index for natural gas generally ranges from 1,250 to 1,440 BTU/scf (44.6-53.6 MJ/Nm3); whereas typical Wobbe Indices for propane and butane are respectively about 1,882 BTU/scf (70.1 MJ/Nm3) and 2,251 BTU/scf (83.8 MJ/Nm3). Gaseous fuels having a Wobbe Index outside these ranges cannot be easily substituted for the aforementioned specified fuel without burdensome design modifications to the burner. By manner of explanation, burners configured for commercial appliances are typically designed for a fuel of specific Wobbe Index, for example, the Wobbe Index of natural gas. In a “partially-aerated” burner the natural gas is pre-mixed with a gaseous oxidant, such as air, and fed at an acceptable velocity to an orifice of the burner, where the mixture is ignited and burned as in a premixed diffusion flame combustion. Substituting a gaseous fuel having a lower Wobbe Index for natural gas results in a lower thermal input (or lower “firing rate”) into the appliance, proportional to a ratio of the two Wobbe Indices (i.e., ratio of the Wobbe Index of the gaseous fuel substitute to the Wobbe Index of natural gas). In order to compensate for the lower firing rate, the diameter of the orifice can be modified to allow more flow for a given pressure. This modification will increase the volumetric flow of fuel through the system and allow a higher firing rate with the lower Wobbe Index fuel. Partially-aerated burners typically include shutters to allow adjustment of premix flow into the burner orifice, which provides for some interchangeability of fuels of similar Wobbe Index, such as from natural gas to propane. The resulting flame, with appropriate level of premix (typically, 25-50 percent of air required for stoichiometric reaction), then with the appropriate addition of secondary air via diffusion at the orifice of the burner (resulting in total air flow of 40-80 percent in excess of stoichiometric) will result in a stable and clean (i.e., low emission, low particulate) flame. Disadvantageously, gaseous fuels with a very low Wobbe Index, for example, below 1,000 BTU/scf (<37.3 MJ/Nm3) cannot be accommodated with the typical levels of adjustability built into commercial burners. Significantly higher velocities through the burner inlet entrain significantly higher quantities of air, which causes a lean condition. The resulting flame is highly unstable and difficult to ignite.
As a further disadvantage, a gas-fired burner cannot be operated directly on a liquid fuel. Transport and combustion of liquid fuels require entirely different design mechanisms from those used with gaseous fuels. To be specific, liquid fuels are susceptible to gravitational factors, require vaporization prior to mixing with air, and may be chemically incompatible with seals and other materials inside the appliance. As another disadvantage, during start-up when a flame and burner parts are cold, a liquid fuel is prone to incomplete combustion, which in turn produces smoking and hydrocarbon emissions. Smoking tends to decrease as a burner flame and parts warm-up and substantially diminishes when the burner is operating properly at steady-state conditions. Smoking and hydrocarbon emissions are highly undesirable at any point of operation, especially during a cold start-up, and even more so when the burner is intended for use in cooking appliances, such as an oven.
U.S. Pat. Nos. 7,976,594 and 8,795,398 disclose an apparatus and method for reforming a liquid distillate fuel, such as kerosene, diesel, and JP-8. The apparatus comprises an ultra-short-channel-length metal substrate provided in a coiled configuration having a radial flow path from an inner diameter to an outer diameter. Supplies of liquid fuel and oxidant, typically air, are taught to be contacted with the coiled substrate; and catalytic partial oxidation (CPOX) occurs therein to produce a gaseous reformate comprising hydrogen and carbon monoxide.
U.S. Pat. Nos. 7,913,484 and 8,387,380 disclose a catalytic burner comprising an ultra-short-channel-length metal mesh substrate. The burner is taught to be employed for full combustion of a liquid distillate fuel to produce thermal energy, which is captured as heat in the head of a Stirling engine.
Patent application publication US 2011/0165300A1 discloses a cooking appliance constructed with a catalytic burner comprised of an ultra-short-channel-length metal substrate, which is conductively contacted by means of a heat spreader to a heat conductive surface. The burner is operated under full combustion conditions to produce thermal energy, which is captured on the conductive surface for cooking applications.
The art would benefit from discovery of an apparatus and a method of operating a gas-fired burner, for example, a natural gas-fired burner, on a liquid fuel, such as those liquid distillate fuels used for propulsion purposes. Such a method should provide for an essentially smokeless start-up from cold conditions. With such a discovery, the burden and cost of transporting two fuels, i.e., a liquid propulsion fuel and a non-propulsion gaseous fuel, to remote locations would be avoided. Only one liquid fuel would be provided for both propulsion and non-propulsion applications; and the gaseous fuels commonly used in static gas-fired burners would be employed as a matter of choice, rather than necessity. The benefits would be particularly advantageous in logistics and camp operations. Moreover, an essentially smokeless start-up would render the apparatus and method suitable for cooking applications, where smoking and release of hydrocarbon emissions are highly unacceptable.